1. Field of the Invention
The present invention relates generally to additives for fracturing fluids. More specifically, the invention is a dry, granulated blend comprising a crosslinker and a delay agent.
2. Description of the Prior Art
In the recovery of hydrocarbons from subterranean formations it is common practice, particularly in formations of low permeability, to fracture the hydrocarbon-bearing formation, providing flow channels. These flow channels allow the oil or gas to reach the wellbore so that the oil or gas may be pumped from the well.
Water-based hydraulic fracturing fluids usually contain a hydratable polymer that acts to thicken the fracturing fluid and may be further thickened by chemically crosslinking. Such a polymer typically is presented in a powder form, or in a slurried form in a hydrocarbon such as diesel, and is hydrated upon the surface of the ground, for example in a batch mix liquid operation in large mixing tanks for a significant period of time, and then mixed with other liquid additives of various types using large expensive equipment. After hydration, the polymer is typically crosslinked to further thicken the fluid and improve its viscosity at elevated temperatures often encountered in the fracture, so it can carry proppant into the fracture once it is pumped into a wellbore below the ground surface. Natural polymers include polysaccharides, such as guar and derivatives of guar such as hydroxypropyl guar (HPG), carboxymethylhydroxypropyl guar (CMHPG), carboxymethyl guar (CMG), or hydrophobically modified guar. Borate, zirconium and titanium containing crosslinking agents typically are used. Both borate and organometallic crosslinking agents offer advantages depending upon the fluid performance and cost requirements of the particular fracturing treatment.
Numerous chemical additives such as antifoaming agents, acids or bases, or other chemicals may be added to provide appropriate properties to the fluid after it is hydrated. Other additives commonly included in fracturing fluids include viscosity stabilizers, activators for crosslinking, shear recovery agents, hydration enabling agents and clay stabilizers. Generally, a viscosity stabilizer is an additive used to retard the polymer degradation from the effects of temperature, shear and iron exposure. A clay stabilizer prevents the swelling or migration of the clays in the formation.
Polymer based hydraulic fracturing fluids are well known in the oilfield services industry. These fluids are routinely used to treat and fracture subterranean formations to increase production from these formations. Typically, the fluid is prepared or mixed at the surface by combining a number of liquid additive streams with a hydrated polymer fluid. The fluid is then pumped downhole with sufficient pressure to accomplish the treatment. In certain cases, the fluid may be used to transport proppant or other additives to the formation. The viscosity of the fluid is often an important consideration in the job design. The fluid must have sufficient viscosity to transport any included solids, such as proppant; however, it cannot be so viscous that it cannot be economically pumped downhole.
Crosslinkers are commonly used to increase the viscosity of polymer based fracturing fluids. The crosslinker chemically connects or bonds the polymer chains in the fluid, thereby increasing viscosity. Well known crosslinkers of polymer fluids include boron-, zirconium- and titanium-containing compounds. In many cases, the use of a crosslinker alone causes a very rapid increase in the viscosity of the fluid and may present significant problems in terms of handling and pumping the viscosified fluid (i.e., the amount of horsepower required to pump the highly viscous fluid downhole is greater than that typically provided at the jobsite). To alleviate this problem, the crosslinking of the polymer can be delayed for a predetermined time. In this way, the fluid does not reach its full viscosity until it is downhole. Delay agents are commonly combined with the crosslinker prior to mixing the crosslinker with the polymer fluid.
Delay of the crosslinking mechanism of the polymer can be achieved using different means. One method is to physically trap or sequester the crosslinker inside a capsule that will dissolve with time under certain conditions of temperature, pH, pressure etc. Alternatively, the crosslinker may be bound to or reacted with another chemical (i.e., a delay agent). The release from this chemical delay agent will also be a function of time, temperature and relative concentrations of the crosslinker and delay agent. The delay in the crosslinking reaction is due to a ligand exchange between the crosslinker, the delay agent and the polymer. In simplified terms, the delay is determined by the time required by the crosslinker to “escape” from the delay agent and crosslink the polymer.
Although the fluid additives, including crosslinker and delay agents, are typically provided in liquid form, it is known that certain of these additives or in some cases the additives and polymer may be provided in a dry form. For instance, U.S. Pat. No. 5,372,732 (Harris), teaches a dry, granulated, delayed crosslinking agent. A borate crosslinker is combined with a liquid polysaccharide solution to produce a fluid containing highly crosslinker polymer. This solution is then dried and granulated and may then be added to a polymer fluid.
Another example of the use of a dry blend of polymer and additives is described in U.S. Pat. No. 5,981,446 (Qiu). A dry blended particulate composition which includes the polymer as well as a number of other additives is prepared and added to water to produce a fracturing fluid. The specific composition of the fluid must be predetermined and generally cannot be changed at the wellsite. This loss of flexibility can present significant problems in terms of job completion. Decreased flexibility can also be a significant concern during the design of the job. For example, in a typical fracturing job, initiation of the fracture is accomplished using a linear (non-crosslinked) fluid that does not contain proppant. Once the fracture is formed, the fluid is crosslinked and proppant is added. Clearly, two different blends would be needed in this case, one containing a crosslinker, the other one not containing the crosslinker.